Conventional DVD and CD servo applications use a wide range clock frequencies to accommodate various media sizes and various speeds of operation. If a servo and RF path need to cover speeds ranging from a 4× CD to a 5× DVD, appropriate clock frequencies need a range from 15 Mhz to 135 Mhz. Such a clock also needs to keep a reasonably low and constant Kvco coefficient (i.e., Kvco=100%*Fvco) for appropriate noise immunity and timing loop bandwidth control. Conventional approaches have many difficulties implementing such a wide frequency range oscillator using CMOS technology due to the low Gm of the MOS transistor.
One conventional CMOS ring oscillator is disclosed by John Meneatis and Mark A. Horowitz in the December 1993 JSSC. Such an approach has been widely used for a CMOS Voltage Controlled Oscillators. Such a CMOS ring oscillator experiences obstacles when trying to provide a wide frequency range along with a low Kvco coefficient (Kvco=delta Fvco/delta Vc; Vc=voltage input to the VCO). The current of the ring oscillator delay cell needs to be increased in square relation to the oscillation frequency. In particular, Fvco=sqrt (2*k*Id)/Cload, where k=constant, and Id=current on the delay cell. The reason for the square relationship (i.e., the frequency Fvco does not increase linearly with the current) is that the voltage swing amplitude on the delay cell load made by a CMOS transistor is enlarged as the current Id becomes larger for a higher frequency operation, where Fvco=Id/(Cload*Voltage Swing). In order to get a 10× frequency range on the VCO, the current on the delay cell needs to be extended about 100 times with a same control voltage at the VCO input. Such a 100 times current range is not practical for a CMOS analog design. Even if such a design was possible, such a design would consume a large amount power when operating at a high frequencies.
For a wide frequency range application, another conventional approach for implementing a CMOS VCO has been disclosed by Ian A. Young in November 1992 JSSC. Such a VCO system 10 is shown in FIG. 1. Such a CMOS VCO has a problem as the frequency of oscillation increases. In such a system, the control voltage (i.e., the input of the VCO) is converted to a current for delay cells in the replica cell. A Voltage Controlled Resistor (VCR) load in the replica cell 12 can only match the resistor value in the delay cell, since the replica cell 12 is working in a static (or DC) operation. When the VCO 12 is generating a frequency for oscillation, the system 10 is not operating in a DC mode. In the real operation of the VCO 10, all of the capacitors on the output of the oscillator delay cell start to contribute for the output impedance. In the static DC replica cell, a bias current is generated with a resistor value that excludes the capacitors loading. The amplitude swing of delay cells is different (smaller) than the intended swing in the replica cell. The difference will be exaggerated with process corner variations. In particular, the frequency targeted with a certain input control voltage will have a wide variation from chip to chip or wafer to wafer. Also, the system 10 will not have a constant value Kvco because of a non-linearity of the parasitic capacitance on the output of the delay cell.
Another conventional VCO has been introduced by Mihai A. Margarit et al. in June 1999 JSSC. An LC tank (inductor and capacitor tank) oscillator is disclosed. In this architecture, the frequency of the LC oscillator is controlled by a value of a capacitor. The value of the capacitor can be changed by a varactor diode with a control voltage or by switching a number of capacitors on the LC resonator. In general, an integrated LC resonator is difficult to get a wide frequency range and a low frequency of oscillation. Such a system has difficulty being used in CD or DVD application. In such an architecture, an automatic amplitude control loop (CAAC) controls a current Itail to maintain a certain oscillation swing amplitude. The AAC circuit is not using a voltage controlled resistor (VCR) to a load resistance value (which is used in Ian Young's ring oscillator and in this invention). The current ITAIL is different depending on the parasitic resistance on the LC tank. The power consumption of the VCO has a variation depending on the parasitic parameter.